1. Field of the Invention
This invention relates to optical finders for still cameras, electronic cameras and video cameras.
2. Description of the Prior Art
Two types of optical finders are known: one is the external type widely used in leaf shutter type cameras; the other is the through-the-lens (TTL) type used in the single lens reflex cameras, 8 mm cameras and cine cameras. Most high quality cameras employ a TTL finder because it is superior to the external type in avoiding parallax between the actual and apparent fields of view. Many video cameras employ an electronic viewfinder for reproducing an image using output signals from an image pickup tube. Compared to the optical finder, the electronic finder has advantageous aspects because it is susceptible to monitoring. Drawbacks to the electronic finder are poor accuracy of distance measurement due to low resolving power, far larger bulk and size, and high consumption of electrical energy. Therefore, it is debatable whether the optical or electronic finder is superior. In either case, however, higher brightness and greater image magnification are desirable.
The TTL optical finders may be further divided into a primary imaging type used in the single lens reflex camera and another or secondary imaging type used in the 8 mm camera or cine camers. In the primary imaging type, a first image at a plane equivalent to a prescribed focal plane of the photographic lens, for example, the film plane, is viewed through an eyepiece, so that the image magnification is determined by the focal lengths of the photographic lens and the eyepiece. On the other hand, the finder of the secondary imaging type is constructed with a first image plane equivalent to the focal plane of the photographic lens, a second image plane conjugate to the first image plane with respect to an optical system therebetween and an eyepiece through which an image on the second plane is observed.
FIG. 1 illustrates an example of a prior known finder of the primary imaging type used in the single lens reflex camera. Light entering and emerging from a photographic lens 1 is reflected upward by a quick return mirror 3 positioned at an angle of 45.degree. to an optical axis 2 to form a first image on a focusing screen 5 in equivalent position to a film plane 4. Light from the image on the focusing screen 5 is successively reflected from two roof type reflective faces a1 and a2 and a front face "b" of a pentagonal roof type prism 6 to an eyepiece 7, and reaches a pupil 8. In this case, since the focal length of the eyepiece is restricted because of the air reduced length of the optical path from the first image plane through the pentagonal roof type prism to the front principal point of the eyepiece, a much desired decrease is not possible. Therefore, it has the drawback that the image magnification is low.
FIG. 2 illustrates an example of the prior known finder of the secondary imaging type used in the 8 mm camera or cine camera. An afocal light beam emerging from a zoom section 11 of a photographic lens system enters a half prism 13 and is split into two parts by a reflective surface 14 of the half prism 13 positioned at an angle of 45.degree. to an optical axis 12 of the photographic lens. One part goes upward, and the other part passes through a relay lens 22 to focus on a film plane 23. The light beam moving upwardly from the reflective surface 14 is focused by an objective lens 15 onto a first image plane 16. Light from the image on the first plane 16 is reflected by a total reflection mirror 17 to parallel the optical axis 12 and is focused by an erector lens 18 onto a second image plane 19. Light from this image on the second plane 19 passes through an eyepiece 20, reaching a pupil 21.
The secondary imaging type finder can include a lens system (in the finder of FIG. 2, lens 18) for enlarging the first image to the second image for the purpose of increasing the image magnification. Additionally, since the focal length of the eyepiece is not subject to restriction by the air reduced length of the optical path of the pentagonal roof type prism as in the example of FIG. 1, the image magnification can be further increased by shortening the focal length of the eyepiece. However, there is the drawback that the finder optical system of FIG. 2 has an increased complexity of structure, physical length, bulk and size. In the system of FIG. 2, where the half prism for splitting off an optical path of the finder is positioned intermediate the physical length of the photographic lens, the drawback is not great. However, when the half prism is positioned behind the photographic lens just in front of the film plane, the finder system projects rearward of the film plane to a great degree. Accordingly, the camera as a whole becomes very large and extremely awkward. Thus, such a finder is unsuitable for use in the single lens reflex camera. Particularly, in interchangeable lens-equipped cameras, it is very difficult for the optical path of the finder to emerge at an intermediate point in the photographic lens without requiring a very elaborate and troublesome structure to interchange the photographic lens at the front assembly of lens elements.